This invention relates to head-up optical display devices and more particularly to on-axis display devices using a single block of transparent material.
Typically, the prior art head-up display devices as represented for example by Kirschner U.S. Pat. No. 3,816,005, assigned to the assignee of this application, utilized an "off-axis" type device to provide a compact display for use in vehicles and aircraft. However, the off-axis techniques tend to impose limitations on the viewing zone, that is, the eye position relative to the optical combiner, resulting from the fact that the observer's line of sight is offset somewhat from the optical axis of the collimating mirror. In the off-axis devices this zone is rather restricted due to the narrow area where the light rays are adequately collimated. The optical distortions or minor inaccuracies of the off-axis type combiners are considered acceptable for such applications as aircraft landing aids but for applications with more precise requirements, such as sighting systems for weapons, significantly greater accuracy is required along with a larger field of view. In addition, the off-axis systems often required an image-projector location that interferred to some degree with the observer's lower field of vision.
Many of these disadvantages can be eliminated by using an "on-axis" type display system where the observer's eye is aligned with the optical axis of the mirror. An example of such a system is provided in Creighton U.S. Pat. No. 2,490,747. The prior art on-axis systems can be broken down into two general types: a reflective system using a spherical collimating mirror and a refractive system using collimating lenses and a combining plate. Schaefer U.S. Pat. No. 3,547,522 provides an example of a prior art reflective system and Searle et al U.S. Pat. No. 3,679,297 provides an example of a refractive system. In practice the reflective systems, as shown in Schaefer, have not been practical in aircraft because the relative orientation of the various elements such as the image generator and mirror are subject to significant distortion due to the vibration and strain inherent in most aircraft or vehicle environments. However, rigidizing the structure to overcome the effects of vibration in an aircraft, for example, inevitably requires structural elements that result in an undesirable obscuring of the pilot's view.
With respect to the refractive systems such as Searle et al, the optical properties of such systems that make use of separate optical elements, result in head-up display systems that have very significant disadvantages for combat aircraft. For example, it is considered desirable to have a field of view of at least 20.degree. in head-up display used for combat aircraft weapons systems, but cockpit space constraints, such as instrument location and the requirement that the display apparatus be located outside of the pilot's ejection envelope, has resulted in refractive systems where it was necessary for the pilot to move his head up to 4 inches in each direction in order to see the full 20.degree. field of view. This is a very serious disadvantage in combat aircraft that are subject to large "g" forces.